The demand for wireless data services, such as text messaging (SMS), multi-media messaging (MMS), mobile video and IPTV, demanding higher bandwidth is growing quickly. The third generation partnership project (3GPP) is developing the third generation mobile systems based on evolved GSM core networks. The radio access technology UMTS terrestrial radio access (UTRA) and has come up with a new orthogonal frequency division multiple access (OFDMA) based technology through the long term evolution (LTE) work, which provides a very efficient wireless solution. The OFDMA based air interface is often referred to as the evolved UMTS terrestrial radio access network (E-UTRAN).
During initial access, the user equipment (UE) seeks access to the network in order to register and commence services. The random access (RA) serves as an uplink control procedure to enable the UE to access the network. Since the initial access attempt cannot be scheduled by the network, the RA procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, it has been decided to separate the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronization, from the transmission of user data.
The RA procedure serves two main purposes:                It lets the UE align its uplink (UL) timing to that expected by the eNode B in order to minimize interfering with other UEs transmissions. UL time alignment is a requirement in E-UTRAN before data transmissions may commence.        It provides means for the UE to notify the network of its presence and enables the eNode B to give the UE initial access to the system.        
The so-called Physical Random Access Channel (PRACH) is a UMTS uplink common physical channel, i.e. it is shared between all user equipments within a cell. Each user equipment in the cell utilizes the PRACH to send signaling information such as a call origination requests to the E-UTRAN and, if necessary, a small amount of user data, such as short messages, alphanumerical texts, and so on.
The PRACH configuration (see table below) is a parameter that needs to be set in each cell.
TABLE 1PRACH configurations in LTE showing number of RACHopportunities per 20 ms for each configuration.PRACHSystem frameNRACH perconfigurationnumberSub-frame number20 ms0Even111Even412Even713Any124Any425Any726Any1, 647Any2, 748Any3, 849Any1, 4, 7610Any2, 5, 8611Any3, 6, 9612Any0, 2, 4, 6, 81013Any1, 3, 5, 7, 91014Any0, 1, 2, 3, 4, 5, 6, 7, 8, 92015Even91
An example depicting the PRACH configuration no. 6 according to table 1 is illustrated in FIG. 2, where the time and frequency configuration of the PRACH, the physical uplink shared channel (PUSCH), and the physical uplink control channel (PUCCH) in the LTE uplink is shown. In this example, two RA opportunities with 1 ms length exist in each frame, which means four RA opportunities per 20 ms. According to 3GPP, the bandwidth of a random access opportunity is 1.08 MHz which corresponds to 6 resource blocks (RB).
If the physical RACH configuration contains too few RACH opportunities then the UEs will often collide on the RACH. A RACH collision occurs when two or more UEs transmit on the RACH simultaneously using the same RACH preamble. In that case the eNB cannot separate between the two transmitted signals and at most one of the UEs can succeed with the RACH transmission attempt. Also, in addition to the RACH collisions, the load on the RACH in a cell may become too high, causing power limited UEs to experience an interference level on the RACH that they have difficulties to overcome. Since RACH preambles derived from different root sequences are non-orthogonal, each UEs transmitting a RACH preamble will cause interference to other UEs that have selected another RACH preamble derived from another root sequence. Thus, too few RACH opportunities in a cell cause problems with RACH collisions and may also result in a high RACH interference level.
If the RACH configuration contains too many RACH opportunities, then obviously the above mentioned problems would be solved. However, the RACH uses the same physical uplink resource as the PUSCH. Hence each time-and-frequency resource spent on RACH reduces the amount of time-and-frequency resources that can be spent on the PUSCH. There is clearly a trade-off here between RACH capacity and PUSCH capacity.
Traditionally, manual cell planning procedures are used to set the RACH configuration parameter. It is an objective of the disclosed invention to enable automatic tuning of RACH related parameters in a way that allows for a trade-off between RACH load and user-plane load.